STREET TREES. 4] 
reach maturity under normal conditions form very large oval-headed 
handsome trees, but under the artificial conditions existing in cities 
it is necessary to prune them quite severely when young to remove 
the long vigorous growths and make the heads more compact. This 
pruning stimulates more vigorous growth, which must be removed or 
they will form long branches with heavy tops, that are especially 
hable to be injured by windstorms. The more they are pruned the 
greater the tendency to an undesirable form of growth. They begin 
dropping their leaves early in the summer and lose them very early 
in the autumn. Their root growth is especially vigorous, so that 
they are lable to make trouble in sewers in the manner already 
mentioned. It is largely on this account that many cities prohibit 
the planting of these trees. Except in regions 6, 7, or 8 or in loca- 
tions where smoke and fumes in the air prevent the growing of 
other trees, they should not be planted. 
The Lombardy poplar? is a tall columnar tree adapted for use on 
very narrow streets (fig. 14). It is short lived in many places, due 
largely to the European poplar canker, but otherwise is a satisfactory 
tree for these conditions in all parts of the United States. The trees 
may be planted as close together as 30 feet. _ 
None of the other poplars have much to recommend them for 
street planting. 
RUBBER TREE. 
The rubber tree? is a large-headed handsome evergreen, suitable 
for regions 3 and 5 and the southern parts of regions 2 and 13 when 
the use of an evergreen tree is warranted. 
SILK OAK. 
The silk oak,? or Australian fern, is a large, handsome tree that 
succeeds well in regions 2, 3, and 13; also in region 5 if provided 
with a reasonable amount of moisture, as it stands drought remark- 
ably well. It is covered in early summer with orange-colored 
flowers. 
SWEET GUM. 
The sweet gum‘ is adapted to regions 11, 12, and 13, especially 
on sandy lands. It forms an oval-headed, handsome tree with star- 
shaped leaves that assume a particularly brilliant hue in the autumn. 
It is better adapted to suburban conditions than to the heart of a 
city. Toward the northern limits of its successful cultivation it is 
difficult to transplant, while in the warmer sections of the country 
it can be moved with comparative ease. It should be transplanted 
only in the spring. 
1 Populus italica (Du Roi) Moench. 3 Grevillea robusta A. Cunn. 
2 Ficus elastica Roxb. 4 Liquidambar styraciflua L. 
